Updating By Subtracting?

Hi,
I am currently updating a column in my database table with the value typed into a text box. But it is updating with the new value, rather than taking away the value typed in from the present value of the column.
1. How would i subtract the value typed in away from the current column value?
2. I guess I need to check the column value is above 0 before updating, then rebind the control to show new values after the update has happened, incase two people are using it at once. How would this be achived?
 
Thanks for any help!!
Jon 
 
 P.S.
My code is:
private bool ExecuteUpdate(int quantity)
{
  SqlConnection con = new SqlConnection();
  con.ConnectionString = "Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\ASPNETDB.MDF;Integrated Security=True;User Instance=True";


  con.Open();

  SqlCommand command = new SqlCommand();
  command.Connection = con;
  TextBox TextBox1 = (TextBox)FormView1.FindControl("TextBox1");
  Label labname = (Label)FormView1.FindControl("Label3");
  Label labid = (Label)FormView1.FindControl("Label13");

  command.CommandText = "UPDATE Items SET Quantityavailable = @qty WHERE productID=@productID";
  command.Parameters.Add("@qty", TextBox1.Text);
  command.Parameters.Add("@productID", labid.Text);
 command.ExecuteNonQuery();

  con.Close();
  return true;
}


    private bool ExecuteInsert(String quantity)
    {
        SqlConnection con = new SqlConnection();
        con.ConnectionString = "Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\ASPNETDB.MDF;Integrated Security=True;User Instance=True";

        con.Open();

        SqlCommand command = new SqlCommand();
        command.Connection = con;
        TextBox TextBox1 = (TextBox)FormView1.FindControl("TextBox1");
        Label labname = (Label)FormView1.FindControl("Label3");
        Label labid = (Label)FormView1.FindControl("Label13");

        command.CommandText = "INSERT INTO Transactions (Usersname)VALUES (@User)"+
          "INSERT INTO Transactions (Itemid)VALUES (@productID)"+
          "INSERT INTO Transactions (itemname)VALUES (@Itemsname)"+
          "INSERT INTO Transactions (Date)VALUES (+DateTime.Now.ToString() +)"+
          "INSERT INTO Transactions (Qty)VALUES (@qty)"+
        command.Parameters.Add("@User", System.Web.HttpContext.Current.User.Identity.Name);
        command.Parameters.Add("@Itemsname", labname.Text);
        command.Parameters.Add("@productID", labid.Text);
        command.Parameters.Add("@qty", TextBox1.Text);
        command.ExecuteNonQuery();

        con.Close();
        return true;
    }





protected void Button2_Click(object sender, EventArgs e)
{
  TextBox TextBox1 = FormView1.FindControl("TextBox1") as TextBox;
  ExecuteUpdate(Int32.Parse(TextBox1.Text) );
}

protected void Button2_Command(object sender, CommandEventArgs e)
    {
        if (e.CommandName == "Update")
        {
            SqlDataSource1.Insert();
        }
    }
}